NIA, Tandojam

June 09  - 15, 2003 


Soil is no doubt an essential component of an agricultural system; therefore, the maintenance of soil quality by proper management is important for successful crop growing. While soil provides with the essential requirements like water and nutrients, changes in the basic soil reactions through excessive cultivation and degradation of the soil conditions for supporting crops do take place constantly. Loss of soil quality therefore reduces crop production. The challenge lies in the development of technological innovations to combat the food inadequacy for the expanding population, for the decades ahead. The science of food production demands adequate inputs, and at the same time, the environment has to be protected through proper management. Foliar application of nutrients nevertheless, is no substitute for soil fertilization, and yet, it can be alternative where crop needs are critical and soil application is expensive, time-consuming and many times ineffective.

The leaves of plant are capable of absorbing nutrients supplied in a liquid or aqueous medium. This capacity is exploited in many agronomic practices like application of inorganic nutrients, growth regulators and herbicides, for the purpose of enhancing crop production. Foliar supply of nutrients have many advantages over the root-feeding. There have been considerable interest in the practical use of this technique as also several accomplishments not only in the understanding of the mechanisms involved in foliar uptake, but also in the development of chemical and surfactants or the greater effectiveness.

Several inorganic nutrient elements are generally required by plants for their growth and development. Presently, 25 per cent of these have been identified. Plant roots are the organs which have two important roles, for providing anchorage and for abstracting water and nutrients from the soil. The root and shoot of the plants are mutually dependent. Thus, the shoot receives the inorganic nutrients from the root, the metabolites from the shoot are translocated to the root. The medium for plants contains all the factors/necessary for growth-water, carbon dioxide, inorganic salts and diffuse sunlight. All the plant parts are able to carry on the two vital processes the absorption of solutes and the photosynthesis.

The evidence that the leaves could be feed with inorganic salts effectively came from a number of scientists in the last century. Reports on the beneficial effects of foliar feeding with compounds containing N, Mn, Ca and B on several crops appeared during the next few decades. Urea sprays for N were practice with many crops in several countries. The introduction of radioisotopes in the early 1950s marked the beginning of our understanding of foliar uptake of inorganic nutrients and organic substances. The need to exploit the capacity of plant leaves to absorb inorganic nutrients has increased greatly for a number of reasons (i) adverse soil conditions, which favour fixation of nutrients and thus render many essential ones unavailable for root absorption (ii) root absorption is slow for some elements and also results in poor translocation (iii) relatively large amounts of fertilizers are required for root supply and heavy application loads to soil-water pollution. Foliar supply of nutrients can result in increasing the photosynthetic efficiency and it is possible to modify the physiology of the leaf.

Out knowledge on the mechanism of foliar uptake cuticular penetration, absorption by the leaf cells, cell-to-cell transport within the leaf and the transport out of the leaf to other regions has widened in the last few years. Furthermore, there has been a revival interest in this field, both for basic research and the practical application.

MECHANISM OF FOLIAR UPTAKE: The inorganic nutrient elements given in the form of aqueous sprays have to be first absorbed by the plant leaf before translocation to other ports. The mechanism of absorption by the leaf cells has been studied using leaf disks and leaf slices. Light is found to enhance the absorption. The relative mobility of various elements was studied as early as in 1957, with the aid of radioisotopes and has been grouped into 3 categories based on their rate of transport out of the leaf. Since then, the techniques of application and measurement of transport have been refined by several workers and based on these, the relative mobility of the elements can be grouped as follows: i) Freely mobile: N, P, K, Rb, Na, Mo; ii) Partially mobile: Fe, Mn, Zn, Mo, Cu, B; iii) Relatively mobile: Ca, Mg. There are environmental factors like light, temperature and relative humidity, which directly and indirectly affect foliar absorption. Young leaves have been found to absorb nutrients more effectively than the one mature ones.

Of the major nutrients, N has been extensively used for foliar sprays for many crops and urea has been the source. Urea sprays for fruit trees, and crops like sugarcane and pineapple have been in commercial practice in many countries for a long time. Workers revealed that post harvest urea sprays resulted in greater N uptake and distribution to the new growth in apples. The interest in foliar application of N has been renewed in the last 3 decades, especially for timely supply of N to soybean during the 'grain-filling' period. There are two contrasting features in soybean with regard to N nutrition. Being a leguminous crop, N application depresses N fixation. On the other hand, the N requirement for soybean is one of the highest amongst the field crops.

Foliar nutrition studies have been extensively carried out and application of N sprays to cereals is a regular practice particularly for increasing the longevity of the leaves. Study indicates that foliar sprays with N, P, K and S at the late reproductive stage are superior to early application on the ear length of corn. Foliar sprays of urea during winter are effective in increasing N levels and not the yield of passion-fruit. Foliar application of P and K has been reported to benefit the groundnut crop grown under salt stress. A new role for foliar nutrition is in the regulation of water use efficiency of fruit trees. Sprays with complete nutrients and with KCl help apple trees withstand water-stress. Studies have shown that foliar application of N, P, K and S increase grain yield of corn.